Using the utilization records of an H.M.O. (a prepaid, comprehensive, health service delivery system, the proposed research will carry out descriptive and analytic studies on the utilization experience of a defined population in terms of prevalence and incidence studies on a cohort of the study population with respect to morbidity and services; stochastic distributions of services and morbidities; etiology of disease; provider behavior, and alternative methods of classification of disease.